Wall temperature actuator overrides solar heat gain?

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You can override surface temperatures without using Energy Management System (EMS) objects. Using the SurfaceProperty:OtherSideCoefficients object, you can use a schedule to set surface temperature. However, when you use this object, "solar effects are not accounted for". This will impact the heat balance for that surface, altering the surface heat loss / gain / storage.

If you did use EMS objects, the specific object that allows users to override normal behavior is the EMS Actuator. Users must select from a list of actuators that the EnergyPlus developers included - users DO NOT have free reign to override every single feature of an EnergyPlus model.

That list of available actuators can be found in the EMS Application Guide documentation. Your question relates to the Surface Boundary Conditions actuators within the group of thermal envelope actuators. To override surface temperatures, it gets tricky because you are asked to set values for four parameters: air temperature and heat transfer coefficient for both convection and radiation heat exchange processes. This approach also requires adding a SurfaceProperty:OtherSideConditionsModel object to the input file, which is typically used to calculate boundary conditions for multi-skin facade components. When you use this approach, EnergyPlus doesn't account for solar gains on the OUTSIDE FACE of the wall surface - only convection and radiation.